This study jointly examined illness beliefs held by persons with multiple sclerosis (PwMS) and caregivers in relation to well-being. A group of 68 PwMS and their caregivers completed the Revised Illness Perception Questionnaire, Psychological Well-being Scales, Satisfaction with Life Scale and Positive Affect and Negative Affect Schedule. Findings revealed that PwMS' well-being was primarily predicted by their own illness beliefs, and that also caregivers' well-being was primarily predicted by their own beliefs. Across the two groups, well-being was positively associated with their belief that they understood the disease, and inversely associated with their representations of negative emotions. In addition, among PwMS, well-being was inversely associated with the number of symptoms they specifically attributed to their illness, while among caregivers, well-being was positively associated with beliefs that treatment could control the disease. Based on the study findings, psychoeducational and cognitive-behavioral strategies are suggested to promote well-being among PwMS and caregivers.